Method of ultrasonically embedding bone anchors

ABSTRACT

Bone anchors, after insertion into bone, are designed to remain in place and provide a platform for holding tissue. A method of ultrasonically embedding bone anchors into bone without pre-drilling an anchor receiving hole into the bone has been developed. Also described is an improved bone anchor embedding method utilizing ultrasonic energy to reduce insertion force, whereby the bone anchor and embedding device can be formed very small, (e.g. on the order of 2 to 5 millimeters in diameter). In an embodiment of the present invention, a method of bone anchor embedding utilizing ultrasonic energy is used to insert a bone anchor into a pre-drilled hole, wherein the hole in the bone may be smaller in diameter than the diameter of the bone anchor.

This application is related to the following copending patentapplications: Ser. No. 08/948,952; filed Oct. 10, 1997 [Applicant DocketNo. END-424]; Ser. No. 08/808,637; filed Feb. 28, 1997 [Applicant DocketNo. END-396]; Ser. No. 09/104,612; filed, Jun. 25, 1998 [ApplicantDocket No. MIT-0119]; and Ser. No. 09/104,648; filed, Jun. 25, 1998[Applicant Docket No. MIT-0132], which are hereby incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates, in general, to a method of attaching abone anchor to a bone and, more particularly, to a method of attaching abone anchor utilizing ultrasonic energy.

BACKGROUND OF THE INVENTION

Ultrasonic surgical instruments are finding increasingly widespreadapplications in surgical procedures by virtue of the unique performancecharacteristics of such instruments. Depending on specific instrumentconfigurations and operational parameters, ultrasonic surgicalinstruments can provide increased control while significantly reducingsurgeon effort and fatigue. Ultrasonic surgical instruments can helpreduce surgeon effort during bone shaving, and reduce the force requiredto penetrate tissue, for example, when inserting a trocar through theabdominal wall of a patient.

Ultrasonic trocar obturators may be used to reduce the force necessaryto penetrate the abdominal wall during laparoscopic surgery. Such adevice is disclosed in U.S. Pat. No. 5,449,370, which describes usingultrasonic energy to assist in the insertion of a trocar obturator andcannula into a body cavity through, for example, the abdominal wall of ahuman being, providing access to internal organs or other tissue.

Ultrasonic devices for removing bone cement tubes during prosthesisreplacements have been developed. U.S. Pat. Nos: 5,019,083; 5,413,578;and 5,318,570 describe the need to reduce the force necessary whenremoving bone cement used for prosthesis fixation, and to increase thesurgeons control during the removal process. The ultrasonic devicesdescribed in U.S. Pat. Nos. 5,019,083; 5,413,578; and 5,318,570 aredesigned to loosen, or fracture, cement while leaving the bone intact.

Bone anchors and bone anchor insertion devices have been described in,for example, U.S. Pat. No. 5,522,845, which describes a bone anchor anda bone anchor insertion tool. In U.S. Pat. No. 5,522,845, a bone anchorinsertion tool is used to insert a specially adapted bone anchor into apre-drilled hole. The bone anchor described in U.S. Pat. No. 5,522,845includes a suture placed through a bore in the body of the bone anchor.The insertion device funnels and protects the suture from damage duringthe insertion of the bone anchor into the bone. The bone anchordescribed in U.S. Pat. No. 5,522,845 also includes barbs protruding fromits body. The barbs are used to retain the anchor in the pre-drilledhole. The pre-drilled hole is designed to have a diameter larger thanthe diameter of the maximum cross-section of the bone anchor in order toreduce the insertion force required to insert the anchor into the bone.

Bone anchors, after insertion into bone, are designed to remain in placeand provide a platform for holding tissue. Bone anchors which include awide spaced multiple turn thread on the outer surface of the bone anchorbody have been described in, for example, U.S. Pat. No. 5,411,523. Theteaching of U.S. Pat. No. 5,411,523 provides a strong union, orpurchase, between the bone and the anchor whereby the anchor is drivenclockwise to insert the bone anchor into a pre-drilled hole (i.e. aright-handed screw thread). Counter-clockwise rotation is used todisconnect the bone anchor from the insertion device. Additionally U.S.Pat. No. 5,411,523 describes a bone anchor with a center longitudinalscrew thread hole also containing right-handed screw threads.

Bone anchor insertion devices have also been described in, for example,U.S. Pat. No. 5,662,658, which describes a bone anchor and drivercontained within a sleeve. The sleeve is connected to a handle with anactuator operative from the handle. In U.S. Pat. No. 5,662,658 theactuator is used to move the driver axially and insert the bone anchorinto a previously drilled hole.

It would be advantageous to provide a method of ultrasonically embeddingbone anchors into bone without pre-drilling an anchor receiving holeinto the bone. It would also be advantageous to provide a method ofultrasonically embedding a bone anchor whereby the removal of theultrasonic embedding device from the bone anchor sets the bone anchorinto the bone. It would be advantageous to provide an improved boneanchor embedding method which utilizes ultrasonic energy to reduceinsertion force, whereby the bone anchor and embedding device can beformed very small, (e.g. on the order of 2 to 5 millimeters indiameter). It would further be advantageous to provide a method of boneanchor embedding utilizing ultrasonic energy to insert a bone anchorinto a pre-drilled hole, wherein the hole in the bone may be smaller indiameter than the diameter of the bone anchor.

SUMMARY OF THE INVENTION

A method of ultrasonically embedding bone anchors into bone isdescribed, comprising the steps of providing an ultrasonic bone anchorembedding device including a bone anchor, placing the ultrasonic boneanchor against a bone, energizing the bone anchor embedding devicecausing it to vibrate ultrasonically, embedding the bone anchor intobone, and then removing the bone anchor embedding device from the boneanchor.

In another embodiment, a method of embedding bone anchors into bone isdescribed, including the steps of providing an ultrasonic bone anchorembedding device including a bone anchor, drilling a pilot hole in abone where the pilot hole diameter is less than or equal to the boneanchor diameter, placing the ultrasonic bone anchor in contact with thepilot hole, energizing the bone anchor embedding device causing it tovibrate ultrasonically, embedding the bone anchor into bone, and thenremoving the bone anchor embedding device from the bone anchor.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe appended claims. The invention itself, however, both as toorganization and methods of operation, together with further objects andadvantages thereof, may best be understood by reference to the followingdescription, taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is an isometric view illustrating an ultrasonic bone anchorembedding device in accordance with the present invention, including anultrasonic transducer, waveguide assembly, and ultrasonic bone anchor;

FIG. 1A is a fragmentary sectional view illustrating an ultrasonic boneanchor embedding device in accordance with the present invention,including an ultrasonic transducer, housing, and a proximal portion of awaveguide assembly;

FIG. 2 is an enlarged, fragmentary, isometric view of the distal end ofthe ultrasonic waveguide assembly illustrated in FIG. 1, including anultrasonic bone anchor, suture, and waveguide;

FIG. 3 is an enlarged sectional view taken along line 3--3 of FIG. 2,illustrating a threaded connection between the end of the ultrasonicwaveguide assembly and the bone anchor;

FIG. 4 is an enlarged view of the proximal face of an ultrasonic boneanchor illustrating the location of a threaded bone anchor connector andsuture attachment;

FIG. 5 is an enlarged, fragmentary, isometric view of the distal end ofthe ultrasonic bone anchor embedding device containing an ultrasonicbone anchor approaching a bone;

FIG. 6 is a fragmentary, partially sectioned view of an ultrasonic boneanchor being driven into bone by an ultrasonic waveguide;

FIG. 7 is a fragmentary, perspective, partially sectioned viewillustrating removal of the ultrasonic bone anchor from the ultrasonicbone anchor driver;

FIG. 8 is an enlarged partially sectioned isometric view of anultrasonic bone anchor set in bone with suture extending out of thebone;

FIG. 9 is an enlarged fragmentary isometric view of an ultrasonic boneanchor driver distal end and an ultrasonic bone anchor, illustrating ataper connection between the ultrasonic bone anchor and ultrasonic boneanchor driver;

FIG. 10 is an enlarged fragmentary isometric view of an ultrasonic boneanchor driver distal end and an ultrasonic bone anchor containing barbs,illustrating a taper connection between the ultrasonic bone anchor andultrasonic bone anchor driver.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is an isometric view illustrating an ultrasonic embedding device10 in accordance with the present invention, including an ultrasonictransducer assembly 20, waveguide assembly 16, and ultrasonic boneanchor 30. Waveguide assembly 16 comprises an ultrasonic waveguide 18which may include depth indicators 24, an outer sheath 17, and rotationknob 14. Ultrasonic bone anchor 30, which may include a suture 32, isconnected to ultrasonic waveguide 18 of waveguide assembly 16 by, forexample, screw threads such as those illustrated in FIG. 3.

FIG. 1A is a fragmentary sectional view illustrating ultrasonicembedding device 10 in accordance with the present invention, includingan ultrasonic transducer assembly 20, device housing 12, and theproximal portion of waveguide assembly 16. Device housing 12 includestransducer support ribs 13 which rotatably support transducer assembly20 within device housing 12. Transducer assembly 20 is inserted intodevice housing 12 and attached to waveguide assembly 16 similarly to theultrasonic clamp coagulator apparatus described in U.S. Pat. applicationSer. No. 08/948,952 previously incorporated herein by reference.Ultrasonic transducer stack 60 is located within transducer housing 21by stack mount 62. Velocity transformer 64 is positioned between stack60 and waveguide/stack coupler 66. Wires 23 from transducer cable 22conduct electrical energy to stack 60 from a signal generator (notshown). A suitable ultrasonic signal generator is available from EthiconEndo-Surgery, Inc. as make ULTRACISION® and model GEN01. Waveguide 18,outer sheath 17, and rotation knob 14 are rigidly connected by waveguidepin 15, as described in application Ser. No. 08/808,637 previouslyincorporated herein by reference.

FIG. 2 is an enlarged, fragmentary, isometric view of the distal end ofthe ultrasonic waveguide assembly 16 illustrated in FIG. 1, including anultrasonic bone anchor 30, suture 32, and ultronic waveguide 18. Anchorthreads 26 are positioned on the outer surface of ultrasonic bone anchor30. In the embodiment of FIG. 1, anchor threads 26 are raised regionsarranged in a left handed screw thread. Suture 32 is connected toultrasonic bone anchor 30 and extends from proximal face 33 (Shown inFIG. 3), through suture attachment recess 36 and suture groove 34 inultrasonic waveguide 18. Depth indicators 24 are spaced along the distalend of ultrasonic waveguide 18.

FIG. 3 is an enlarged sectional view taken along line 3--3 of FIG. 2.FIG. 3 illustrates the threaded connection between the distal end ofultrasonic waveguide 18 and ultrasonic bone anchor 30. Anchor threads26, which are arranged in a left handed screw thread, lie on the outersurface of ultrasonic bone anchor 30. Ultrasonic bone anchor 30 containsthreaded connector 28 which may be formed in a right handed screwthread. Threaded connector 28 includes anchor recess 58 in ultrasonicbone anchor 30, female threads 56 formed within anchor recess 58,waveguide stud 52 at the distal end of ultrasonic waveguide 18, and malethreads 54 formed on waveguide stud 52. Ultrasonic bone anchor 30 isshown screwed onto ultrasonic waveguide 18 utilizing threaded connector28.

FIG. 4 is an enlarged view of the anchor proximal face 33 of ultrasonicbone anchor 30. FIG. 4 illustrates the anchor face suture location 35.Anchor recess 58 is centrally located in the anchor proximal face 33.Suture 32 is illustrated attached radially from anchor recess 58 atanchor face suture location 35. Anchor face suture location 35 islocated such that suture 32 lies within suture attachment recess 36(shown in FIG. 2) when ultrasonic bone anchor 30 is attached toultrasonic waveguide 18 (FIG. 3). Suture 32 may be attached toultrasonic bone anchor 30 by standard means known in the art, forexample, gluing.

Operation of the illustrated embodiment of the invention may beunderstood by referring to FIGS. 5-8. FIG. 5 is an enlarged,fragmentary, isometric view of the distal end of the ultrasonicembedding device 10 including an ultrasonic bone anchor 30 which isillustrated approaching a bone mass 38. Bone anchor tip 31 of ultrasonicbone anchor 30 is illustrated in contact with bone mass 38. Suture 32lies within suture groove 34 of ultrasonic waveguide 18 and extendsinside of outer sheath 17 along suture groove 34. Transducer assembly20, shown in FIG. 1, may be activated to drive ultrasonic bone anchor 30into bone mass 38.

FIG. 6 is a fragmentary, partially sectioned view of an ultrasonic boneanchor 30 being driven into bone mass 38 by ultrasonic insertion device10 which is connected to ultrasonic bone anchor 30 by ultrasonicwaveguide 18. Bone anchor tip 31 of ultrasonic bone anchor 30 will bedriven into bone mass 38. As ultrasonic bone anchor 30 is driven intobone mass 38, creating anchor pilot hole 39, depth indicators 24 willindicate to the user the depth of bone anchor tip 31 as measured in, forexample, millimeters. As ultrasonic bone anchor 30 is driven into bonemass 38 by ultrasonic embedding device 10, anchor thread 26 will rotateultrasonic bone anchor 30 and waveguide assembly 16 in acounter-clockwise direction, as illustrated in FIG. 6. (Clockwise isdefined herein as the rotation direction of the hands of a standardanalog clock when viewing ultrasonic embedding device 10 while holdingdevice housing 12 with the distal end of ultrasonic waveguide 18 pointedaway from the observer.) Suture groove 34 will protect suture 32 fromdamage resulting from contact with the interior wall of anchor pilothole 39 in bone mass 38.

Ultrasonic bone anchor 30 is driven into bone mass 38 using mechanicalenergy at ultrasonic frequency, creating anchor pilot hole 39. Boneanchor tip 31, when energized, vibrates reciprocally with a tip peak topeak excursion of, for example, about 100 micrometers. The bone anchortip 31 of the ultrasonic embedding device 10 may have an excursion ofbetween 50 and 300 micrometers, which would correspond to tip peakvelocities ranging from 8.7 meters per second to 52.3 meters per secondat 55,500 Hertz. The reciprocal motion will compact and fragment bonemass 38 and create anchor pilot hole 39 analogous to a jackhammerdriving its tip into cement.

Anchor threads 26 of an ultrasonic bone anchor 30 are preferably nofiner in pitch than one full turn per bone anchor length, and mostpreferably less than or equal to 1/4 turn per bone anchor length. Forexample, a one centimeter long ultrasonic bone anchor 30 wouldpreferably have no more than about one turn per centimeter thread pitch.Non-ultrasonic bone anchors utilize external threads to assist insertionof bone anchors into pre-drilled pilot holes. Finer thread pitches suchas, for example, three to twenty threads per centimeter, require moreturns of a non-ultrasonic bone anchor for a given insertion depth, butreduce insertion force as thread pitch becomes finer. Ultrasonic boneanchors utilize ultrasonic energy to reduce insertion force. Externalthreads of ultrasonic bone anchors 30 are primarily utilized to restrictrotation of the bone anchor within anchor pilot hole 39 during removalof ultrasonic bone anchor 30 from ultrasonic embedding device 10. Thepreferable range for ultrasonic bone anchors 30 is zero to two turns perultrasonic bone anchor 30 length. Finer pitches than two turns perultrasonic bone anchor 30 length may actually be detrimental toultrasonic bone anchor 30 purchase of bone mass 38 by causing anenlarging of bone anchor hole 39 during insertion.

In an alternate embodiment of the invention, rotation of waveguideassembly 16, as a result of the force applied by anchor thread 26 duringembedding of ultrasonic bone anchor 30 into bone mass 38, may be used toindicate depth of embedding of ultrasonic bone anchor 30 into bone mass38. Rotation knob 14 is rigidly connected to ultrasonic waveguide 18,and will rotate with respect to device housing 12 as waveguide assembly16 rotates. Depth indicators 24 are spaced along the distal end ofultrasonic waveguide 18 to indicate how far ultrasonic bone anchor 30has penetrated into bone during placement. Depth indicators mayalternately be located in device housing 12, which utilize the rotationof waveguide assembly 16 or rotation knob 14 to indicate depth ofembedding of ultrasonic bone anchor 30 into bone mass 38.

FIG. 7 is a fragmentary, perspective, partially sectioned viewillustrating removal of ultrasonic bone anchor 30 from the ultrasonicembedding device 10. Removal of ultrasonic bone anchor 30 from waveguideassembly 16 may be accomplished by counter-clockwise rotation ofrotation knob 14. Rotation knob 14 is rigidly fixed to both outer sheath17 and ultrasonic waveguide 18, but is rotatably mounted to devicehousing 12 as described in U.S. patent application 08/948,952 previouslyincorporated herein by reference. Counter-clockwise torque applied tothreaded connector 28 will unscrew waveguide stud 52 from anchor recess58 while forcing ultrasonic bone anchor 30 into bone mass 38 due to theleft hand pattern of anchor thread 26. Suture attachment waveguiderecess 36 at the distal end of ultrasonic waveguide 18 will facilitatedisengagement of ultrasonic bone anchor 30 from ultrasonic waveguide 18without pinching or damaging suture 32. Suture 32 will be pulled alongsuture groove 34 of ultrasonic waveguide 18 as waveguide assembly 16 isremoved from ultrasonic bone anchor 30.

FIG. 8 is an enlarged partially sectioned isometric view of anultrasonic bone anchor 30 set in bone mass 38 with suture 32 extendingout of the bone mass 38. Ultrasonic bone anchor 30 is set in bone mass38, and fixed in place by anchor thread 26. Suture 32 extends out ofbone mass 38 and can be used for attachment to bone mass 38. Anchorpilot hole 39 has been formed in bone mass 38 by driving ultrasonic boneanchor 30 into bone mass 38 using ultrasonic embedding device 10. Threadtrack 41 is illustrated in bone mass 38 on the inner surface of anchorpilot hole 39.

FIG. 9 is an enlarged fragmentary isometric view of another embodimentof ultrasonic bone anchor 30, illustrating a taper connection betweenultrasonic bone anchor 30 and ultrasonic embedding device 10. Ultrasonicwaveguide 18 of ultrasonic embedding device 10 is illustrated containingwaveguide post recess 44, which is tapered as it extends into ultrasonicwaveguide 18 from waveguide distal face 19. Ultrasonic bone anchor 30contains suture attachment post 40 which is also tapered to fit intowaveguide post recess 44. Suture attachment post 40 contains post sutureopening 46 which accommodates insertion of suture 32 through post sutureopening 46 of suture attachment post 40. Suture 32 may lie within suturegroove 34 of ultrasonic waveguide 18 during embedding, or suture 32 maybe inserted into post suture opening 46 of ultrasonic bone anchor 30after ultrasonic bone anchor 30 has been driven into bone mass 38 (FIG.8).

FIG. 10 is an enlarged fragmentary isometric view of a bone anchorultrasonic embedding device 10 distal end and an ultrasonic bone anchor30 containing barbs 42, illustrating a taper connection betweenultrasonic bone anchor 30 and ultrasonic embedding device 10. Referringto FIGS. 8 and 10, barbs 42 are used in place of the anchor thread 26 toanchor ultrasonic bone anchor 30 into bone mass 38. Barb 42 lies withinanchor barb recess 43 of ultrasonic bone anchor 30 during embedding ofultrasonic bone anchor 30 into bone mass 38. Barb 42 may be manufacturedfrom elastic material such that barb 42 pushes out against the innersurface of anchor pilot hole 39 after embedding of ultrasonic boneanchor 30 into bone mass 38, restricting ultrasonic bone anchor 30 frombacking out of anchor pilot hole 39. Barbs 42 or anchor threads 26 willincrease the purchase, or holding strength, of ultrasonic bone anchor 30in anchor pilot hole 39.

Transducer assembly 20 (FIG. 1A), which may also be referred to as ahandpiece, comprises an ultrasonic transducer, preferably apiezeoceramic transducer, for converting an electrical signal, forexample, a 55,500 Hz sinusoidal waveform, into a mechanical longitudinalvibration. A suitable ultrasonic transducer assembly is available fromEthicon Endo-Surgery, Inc. as make ULTRACISION® and model HP051.

Instrument device housing 12 includes rotation knob 14. Waveguideassembly 16 comprises outer sheath 17, ultrasonic waveguide 18, andultrasonic bone anchor 30. Rotation knob 14 is rigidly connected towaveguide assembly 16. A surgeon using ultrasonic embedding device 10may grasp device housing 12 in one hand, and rotate rotation knob 14with either a finger, or the other hand. Rotation of rotation knob 14causes waveguide assembly 16, ultrasonic bone anchor 30, and transducerassembly 20 to rotate within device housing 12.

Embedding of ultrasonic bone anchor 30 may be accomplished by bringingbone anchor tip 31 in contact with an area of bone mass 38 where afastening location is desired. The surgeon will then cause transducerassembly 20 to be energized. Ultrasonic energy will propagate alongultrasonic waveguide 18 and be delivered through ultrasonic bone anchor30 to bone anchor tip 31. Bone anchor tip 31 will be caused to vibratelongitudinally at an excursion of approximately 50 micrometers to 300micrometers at an ultrasonic frequency of, for example, 55,500 Hertz. Asthe surgeon applies a small forward force to device housing 12,ultrasonic bone anchor 30 will penetrate bone mass 38 assisted by theultrasonic embedding force.

Ultrasonic bone anchor 30 may be driven into bone mass 38 with orwithout the need for a pre-drilled hole. Ultrasonic bone anchor 30,having an excursion of between 50 and 300 micrometers, would correspondto bone anchor tip 31 peak accelerations ranging from about 480,000meters per second² to 2,900,000 meters per second² at 55,500 Hertz. Theforce generated at bone anchor tip 31 to bone mass 38 due to the massand acceleration of the bone anchor tip 31 greatly reduces the forcerequired by the surgeon to accomplish the embedding.

Embedding of ultrasonic bone anchor 30 into bone mass 38 without apre-drilled hole is best accomplished using an ultrasonic bone anchor 30manufactured from a hard material such as, for example, Ti-6A1-4V, atitanium alloy. However, ultrasonic bone anchor 30 may also bemanufactured from polymeric materials which are softer than bone.

Embedding of an ultrasonic bone anchor 30 manufactured from a polymericmaterial is best accomplished by pre-drilling the bone mass 38 with ahole slightly smaller than the diameter of ultrasonic bone anchor 30. Asultrasonic bone anchor 30 is driven into bone mass 38 using ultrasonicenergy, ultrasonic bone anchor 30 will be swaged to an optimum size forthe pre-drilled hole. This will result in a stronger anchor attachmentthan if the hole was the same size or larger than the ultrasonic boneanchor 30, as is described in the prior art. Embedding force by thesurgeon to swag ultrasonic bone anchor 30 into bone mass 38 will beminimal due to the ultrasonic force assistance as described above.

Referring to FIGS. 1 and 1A, the acoustic assembly 50 of the ultrasonicembedding device 10 generally includes transducer stack 60 used toconvert electrical energy into mechanical energy, velocity transformer64 used to amplify excursion, ultrasonic waveguide 18 used to transferthe mechanical energy, and ultrasonic bone anchor 30. In order for theacoustic assembly 50 to deliver energy through the ultrasonic boneanchor 30, the transducer stack 60, ultrasonic waveguide 18, andultrasonic bone anchor 30 must be acoustically coupled. The distal endof the transducer stack 60 is acoustically coupled to the proximal endof ultrasonic waveguide 18 preferably by a threaded connection.

The components of the acoustic assembly 50 are preferably acousticallytuned such that the length of each component is an integral number ofone-half wavelengths (nλ/2), where the wavelength λ is the wavelength ofa pre-selected or operating longitudinal vibration frequency f₀ of theacoustic assembly 50, and where n is any positive integer. It is alsocontemplated that the acoustic assembly 50 may incorporate any suitablearrangement of acoustic elements.

The transducer assembly 20 of the acoustic assembly 50 converts theelectrical signal from a signal generator into mechanical energy thatresults in longitudinal vibratory motion of the acoustic assembly 50 atultrasonic frequencies. When the acoustic assembly 50 is energized, avibratory motion standing wave is generated through the acousticassembly 50. The excursion of the vibratory motion at any point alongthe acoustic assembly 50 depends on the location along the acousticassembly 50 at which the vibratory motion is measured. A minimum or zerocrossing in the vibratory motion standing wave is generally referred toas a node (i.e., where motion is usually minimal), and an absolute valuemaximum or peak in the standing wave is generally referred to as ananti-node. The distance between an anti-node and its nearest node isone-quarter wavelength (λ/4).

As illustrated in FIGS. 3, 9 and 10, the ultrasonic waveguide 18 of thesecond acoustic portion is preferably detachably coupled to ultrasonicbone anchor 30. The ultrasonic waveguide 18, including ultrasonic boneanchor 30 acoustically coupled thereto, preferably has a lengthsubstantially equal to an integer number of one-half system wavelengths(nλ/2). The ultrasonic waveguide 18 is preferably fabricated from asolid core shaft constructed out of material which propagates ultrasonicenergy efficiently, such as titanium alloy (i.e., Ti-6A1-4V) or analuminum alloy. It is contemplated that the ultrasonic waveguide 18 canalternatively be fabricated from any other suitable material.

The ultrasonic waveguide 18 may be semi-flexible. The waveguide may beconfigured to amplify the mechanical vibrations transmitted through thewaveguide to the end-effector as is well known in the art. The waveguidemay further have features to control the gain of the longitudinalvibration along the waveguide and features to tune the waveguide to theresonant frequency of the system.

It is important to account for the acoustic property of ultrasonic boneanchor 30 in the ultrasonic embedding device 10. An ultrasonic boneanchor 30 manufactured from a material such as, for example, Ti-6A1-4Vtitanium alloy, acoustically couples to ultrasonic waveguide 18 tobecome part of the resonant acoustic system, having an overall length ofnλ/2. If the acoustic length (the length as measured in wavelengths) ofthe bone anchor is, for example, λ/16, then the overall acoustic lengthof the ultrasonic embedding device 10 is optimally adjusted to(nλ/2)-λ/16.

Embedding devices of 5 mm diameter or less inherently begin to becomemore flexible as their diameters decrease. This flexibility causes themto buckle under compressive load as a surgeon attempts to embed a boneanchor. Utilizing a tuned acoustic assembly 50 in a standing wave modeof vibration allows the embedding of very small (less than 5 mm )ultrasonic bone anchors. As described earlier, ultrasonic waveguideshave the ability to produce significant forces at the tip of ultrasonicbone anchor 30. The dynamic, resonant, standing wave nature of theultrasonic tip force does not significantly contribute to the bucklingload of the embedding device. This allows significant embedding forceapplication without significant surgeon force application to theembedding device that could lead to embedding device buckling.

While preferred embodiments of the present invention have been shown anddescribed herein, it will be obvious to those skilled in the art thatsuch embodiments are provided by way of example only. Numerousvariations, changes, and substitutions will now occur to those skilledin the art without departing from the invention. Accordingly, it isintended that the invention be limited only by the spirit and scope ofthe appended claims.

What is claimed is:
 1. A method of ultrasonically embedding bone anchorsinto bone, said method comprising the steps of:a) providing a boneanchor, wherein said bone anchor comprises:a proximal end including acoupling element; a distal end; an outer surface, including at least oneprotrusion on said outer surface of said body; a central portion betweensaid distal end and said proximal end, said central portion having alength, wherein said protrusion is a thread, said thread having a threadpitch less than one turn per said bone anchor central portion length; b)providing an ultrasonic bone anchor embedding device, wherein said boneanchor is attachable to said bone anchor embedding device and whereinsaid bone anchor embedding device is adapted to transmit ultrasonicenergy therethrough; c) placing said bone anchor against a bone; d)energizing said bone anchor embedding device causing said bone anchorembedding device to vibrate ultrasonically; e) embedding said boneanchor into said bone; and f) removing said bone anchor embedding devicefrom said bone anchor.
 2. A method of embedding bone anchors into boneaccording to claim 1, wherein said bone anchor coupling element includesscrew threads.
 3. A method of embedding bone anchors into bone accordingto claim 2, comprising removing said bone anchor embedding device fromsaid bone anchor with a counter clockwise rotation of said bone anchorembedding device.
 4. A method of embedding bone anchors into bone, saidmethod comprising the steps of:a) providing a bone anchor, wherein saidbone anchor comprises:a body having a bone anchor body length and anouter surface; a proximal end including a bone anchor coupling element;and at least one engagement thread on said outer surface of said boneanchor wherein said engagement thread is formed as a left-handed thread,said engagement thread having a thread pitch less than one turn per saidbone anchor body length; b) providing an ultrasonic bone anchorembedding device, wherein said bone anchor is attachable to said boneanchor embedding device and wherein said bone anchor embedding device isadapted to transmit ultrasonic energy therethrough; c) placing said boneanchor against a bone; d) energizing said bone anchor embedding devicecausing said bone anchor embedding device to vibrate ultrasonically; e)embedding said bone anchor into said bone; and f) removing said boneanchor embedding device from said bone anchor.
 5. A method of embeddingbone anchors into bone according to claim 4 wherein said thread has athread pitch of approximately one quarter turn per said bone anchor bodylength.
 6. A method of embedding bone anchors into bone, said methodcomprising the steps of:a) providing a bone anchor, wherein said boneanchor comprises:a body including an outer surface; a proximal endincluding a bone anchor coupling element; and engagement threads on saidouter surface of said bone anchor wherein said engagement threads areformed as left-handed threads; b) providing a bone anchor embeddingdevice wherein said bone anchor is attachable to said bone anchorembedding device and wherein said bone anchor embedding device isadapted to transmit ultrasonic energy therethrough; c) drilling a pilothole in a bone, said pilot hole having a hole diameter, wherein saidhole diameter is less than or equal to said bone anchor diameter; d)placing said bone anchor in contact with said pilot hole; e) energizingsaid bone anchor embedding device causing said bone anchor embeddingdevice to vibrate ultrasonically; f) embedding said bone anchor intosaid pilot hole; and g) removing said bone anchor embedding device fromsaid bone anchor.
 7. A method of embedding bone anchors into boneaccording to claim 6, wherein said bone anchor coupling element includesscrew threads formed as right-handed threads.
 8. A method of embeddingbone anchors into bone according to claim 7, comprising removing saidbone anchor embedding device from said bone anchor with a counterclockwise rotation of said bone anchor embedding device.
 9. A method ofembedding bone anchors into bone, said method comprising the steps of:a)providing a bone anchor, wherein said bone anchor comprises:a bodyhaving a bone anchor body length and an outer surface; a proximal endincluding a bone anchor coupling element; and at least one engagementthread on said outer surface of said bone anchor wherein said engagementthread is formed as a left-handed thread, said engagement thread havinga thread pitch less than one turn per said bone anchor body length; b)providing a bone anchor embedding device wherein said bone anchor isattachable to said bone anchor embedding device and wherein said boneanchor embedding device is adapted to transmit ultrasonic energytherethrough; c) drilling a pilot hole in a bone, said pilot hole havinga hole diameter, wherein said hole diameter is less than or equal tosaid bone anchor diameter; d) placing said bone anchor in contact withsaid pilot hole; e) energizing said bone anchor embedding device causingsaid bone anchor embedding device to vibrate ultrasonically; f)embedding said bone anchor into said pilot hole; and g) removing saidbone anchor embedding device from said bone anchor, wherein said step ofremoving said bone anchor embedding device from said bone anchor setssaid bone anchor into said bone.
 10. A method of embedding bone anchorsinto bone according to claim 9 wherein said thread has a thread pitch ofabout one quarter turn per said bone anchor body length.